Equine arteritis virus (EAV) is a member of the genus Arterivirus, family Arteriviridae in the order Nidovirales (Cavanagh, 1997), and is the causative agent of equine viral arteritis (EVA) of horses (Doll et al., 1957a). Outbreaks of EVA are characterized by any combination of systemic illness of adult horses, abortion of pregnant mares, interstitial pneumonia of young foals and persistent infection of stallions (Doll et al., 1957a; Doll et al., 1957b; Golnik et al., 1981; Timoney et al. 1986; Timoney et al., 1987; Timoncy et al., 1992; Carman et al., 1988; Vaala et al., 1992; Del Piero et al., 1995; Del Piero et al., 1997). EAV is horizontally transmitted either by aerosol during outbreaks of EVA or venerally via the breeding of an infected stallion to susceptible mares, and vertically through congenital infection of foals born to mares infected late in gestation (Timoney et al., 1987; Timoney et al., 1992; Vaala et al., 1992; Timoney and McCollum, 1993; Glaser et al., 1996).
Dissemination of EAV by fomites such as vehicles, twitches, artificial vaginas and shanks can be an important source of infection in some outbreaks (Collins et al. 1987; Timoney and McCollum, 1988; Timoney and McCollum, 1993). The persistently infected carrier stallion clearly plays an important role in perpetuation and sexual dissemination of EAV. The persistence of EAV in the male reproductive tract is testosterone-dependent (Timoney and McCollum, 1993). It was recently shown that EAV behaves as a quasi-species during persistent infection of carrier stallions, with regular emergence of novel genotypic and phenotypic viral variants (Hedges et al., 1999).
The EAV genome is 12.7 kb and contains 5′ and 3′ untranslated regions and nine functional open reading frames [ORFs; (Snijder and Meulenberg, 1998. Snijder et al., 1999)]. ORFs 1a and 1b encode two replicase polyproteins [pp1a and pp1ab; (de Vries et al., 1997; Snijder and Spaan, 2006; Snijder and Meulenberg, 1998)], and the remaining seven ORFs (2a, 2b and 3-7) encode structural proteins of the virus. These include four membrane glycoproteins GP2 (25 kDa), GP3 (3642 kDa), GP4 (28 kDa) and GP5 (30-44 kDa), respectively encoded by ORFs 2b, 3, 4, and 5, two unglycosylated membrane proteins E (8 kDa) and M (17 kDa) encoded by ORFs 2a and 6, and the phosphorylated nucleocapsid protein N (14 kDa) encoded by ORF7 (de Vries et al., 1992; Snijder et al., 1999; Wieringa et al, 2002).
Prevention and control of EVA in North America is achieved by vaccination of horses with the modified live virus vaccine strain of EAV (ARVAC®, Fort Dodge Animal Health; Moore, 1986). Although the current modified live virus (MLV) vaccine against EVA is safe and efficacious, there is resistance to using it in horses in many countries (e.g. European Union) regardless of the seroprevalence of EAV infection. One of the major concerns is the safety of the current MLV vaccine in pregnant mares, in particular the ability of the attenuated virus to cross the placenta and infects the unborn foal. The vaccine is only recommended for use in stallions and nonpregnant mares. It is not recommended for use in pregnant mares, especially during the last two months of gestation, or in foals less than 6 weeks of age, unless they are at high risk of natural exposure. Furthermore, horses that are vaccinated with the current MLV cannot be distinguished from naturally infected animals. Following the recent multi-state EVA occurrence in the United States there is a strong industry demand for a marker vaccine to distinguish vaccinated animals from the naturally infected animals, as well as to develop a MLV vaccine that is totally safe for use in pregnant mares. Thus, there remains a need in the art for novel means for control of outbreaks of EAV. The advent of recombinant DNA technology has helped to develop new generation vaccines against a number of veterinary pathogens. These include live-vectored vaccines, gene deletion mutants and DNA vaccines.